I would say, her wounds are not physical. It's how student girl sees her real self.
Woman is probably closeted lesbian, ending in marriage with a guy who truly loves her. She is making everyone else happy, but not herself.
Younger woman sees it, so she asked "Why did you get married?" - (when you are not attracted to men).
Married woman is hurting herself having physical relations (sex) with her husband, and bruises what student girl sees, are those wounds. When she hears them "fighting" every night, she hears them having sex. - ("he is just clumsy" shows how she is not enjoying sex with him, having no pleasures at all, so younger girl called it "abusing", putting her on the spot).
After confession, woman rejected her, excusing herself being married.
At the end, we all can see her husband loves his wife, taking her to the hot spring, brightly smiling - (he is in love, for sure). But student girl knows it can't heal the "bruises", and it will sting more. But she can't do nothing. At least, she tried. It's up to married woman, her choice...
I like this manga, because it's a slap to readers how in reality soul hurting would look like physically (when eyes are open to see what is deeply hidden).
last edited at Jul 6, 2019 5:56AM